why not have a web app that allows users to add features then you (the developer) can add a price and time to complete.
any person can pledge any amount. a bar chart shows the progress of each feature.
once total pledges reach your price all users are emailed and to submit there payment. have a bar chart for actual.
once actual reaches your price you can start work.

then you might be able to hire more help and get more feature requests added.
not sure if there is anything already made like this out there.